Method and apparatus for electrolytically treating a board-shaped substrate comprising shielding edge regions of the substrate during electrolytic treatment

ABSTRACT

The apparatus according to the invention is used for electrolytically treating a board-shaped substrate to be treated, preferably printed circuit boards, in a continuous system, through which the item to be treated is guidable in a plane of conveyance in a substantially horizontal direction of conveyance, the apparatus having counter-electrodes ( 2 ), which are situated substantially parallel to one another opposite the plane of conveyance, and screens ( 11 ) for shielding from high current density fields in the edge region of the item to be treated ( 1 ), said screens being disposed between the plane of conveyance and the counter-electrodes, the screens each being in the form of at least two flat portions ( 12,13 ), which are disposed substantially parallel to each other, one portion ( 13 ) of the screens being disposed so as to lie opposite the plane of conveyance, and the other portion ( 12 ) being disposed so as to lie opposite the counter-electrodes, and the screens being mounted so as to be displaceable in a direction ( 20 ) which extends substantially parallel to the plane of conveyance and substantially perpendicularly relative to the direction of conveyance ( 23 ). The usable region of printed circuit board blanks can be extended by this apparatus down to an edge of about 12 mm, where the required tolerance of the layer thickness of the deposited metal cannot be maintained.

DESCRIPTION

The invention relates to an apparatus for electrolytically treating aboard-shaped substrate to be treated, as well as relating to a method ofelectrically shielding edge regions of the item to be treated. For theelectroplating or etching of printed circuit boards and conductor foils,the apparatus and the method are preferably used in continuous systemswith the item to be treated being horizontally or vertically alignedduring its passage therethrough.

Substrates to be treated, of varying widths, are generally produced incontinuous electroplating systems. The anodes in the systems must be sodimensioned that even the widest boards and foils can be treated withoutany problems. If, in these circumstances, relatively narrow boards orfoils are electroplated in the system transversely relative to thedirection of conveyance of the item to be treated, considerably thickermetal layers are produced in the edge regions thereof than in thecenter, because the electrical field lines are concentrated in theseedge regions. A specific tolerance for the metal layer thickness isusually prescribed, so that the region which is usable in practice onthe metallized boards and foils is smaller than the overall widththereof. Only a low tolerance for the metal layer thicknesses ispermissible for the production of printed circuit boards and conductorfoils, which thicknesses result from requirements for the subsequentprocessing of boards and foils. Furthermore, printed circuit boardsgenerally involve expensive materials. In consequence, it is desirableto achieve maximum utilization of the electroplated bare printed circuitboards, which are also called blanks. In consequence, a uniform layerthickness as far as the edge region of the printed circuit boards orconductor foils is to be achieved. A known measure for influencing thelayer thickness in the edge region of the item to be electroplatedresides in electrically screening these edge regions by usingelectrically non-conductive screens.

Apparatus for screening field lines in an electroplating system fortreating printed circuits boards, more especially, are described in DE-P39 37 926 C2. Screens are illustrated in FIGS. 2 and 3, containedtherein, and comprise a plurality of partial screens which areindividually adjustable per se. By rotating the partial screens about anaxis, the screening effect of each individual partial screen can beincreased or reduced. The edge covering can be set in optimum mannerwhen the individual screens are narrow enough and, in consequence, whenthere is a sufficiently large number of these screens.

A disadvantage which has to be tolerated, however, is that there remainsa residual screening over the region of the printed circuit boards whichshould not actually be shielded. Furthermore, the structural outlay forachieving this apparatus is considerable. In consequence, a level screenbetween the anodes and the edge region of the item to be treated ispreferred in practical usage, as illustrated in FIG. 4 in theabove-mentioned publication. With this arrangement of the screens,however, an optimum profile-like screening is not possible, with theresult that the required layer thickness tolerances cannot be achievedin a relatively large edge region on the blank. Moreover, the high levelof technical outlay is also disadvantageous for this arrangement.

The electroplating operation requires strong constructions. Inconsequence, a flat, displaceable screen similar to that known fromJP-A-62/151 593 has proved successful in practical usage. The screensused there are displaced by a linearly acting drive between the anodesand the edge region of the item to be treated.

These and similarly flat screens reduce the concentration of field linesin the edge region of the item to be treated in dependence on thecovering of the item to be treated by the screen and on the spacingbetween the screen and the anode, or respectively the surface of theitem to be treated. The spacing between the anodes and the item to betreated as well as the anodic and cathodic current densities are alsosignificant for the electrolytic treatment. The position of the screenduring the electrolytic treatment constitutes a compromise based onthese parameters, so that the optimum screening has to be determined incomplex experiments. An object is to achieve a large usable area withinthe permissible tolerance range without burning the edges in the eventof high current densities. In the case of fine printed circuit boardsand average current densities, the non-usable edge has a width of about30 mm in practical usage. In the case of current densities up to 15A/dm², the non-usable edge with this screen already has a width of about50 mm.

An electroplating arrangement is described in U.S. Pat. No. 3,862,891and has an electroplating frame and anodes, as well as an electricallynon-conductive shielding screen for distributing the concentration offield lines evenly along the item to be electroplated, which is mountedon an article carrier. The shielding screen comprises a plurality ofscreen portions, which are securedly connected to the arrangement, inthat they are secured on the lateral walls and screw-connected insuitable guides. The screen portions are orientated parallel to oneanother and parallel to the item to be electroplated and the anodes. Thescreen portions protrude partially into the space between the item to beelectroplated and the anodes, whereby the screen portions situatedcloser to the anodes protrude further than the screen portions situatedcloser to the item to be electroplated. It is stated that, with thisarrangement, a uniform electroplating layer thickness can be achieved inthe plane of the electroplating frame.

It is not possible, with this arrangement, to treat items of varyingexternal dimensions electrolytically in optimum manner, since aprescribed tolerance of the metal layer thickness is not achieved, onlyon as narrow an edge region of the item to be treated as possible, whilethe remaining surface of the item to be treated meets the requirementsfor the uniformity of the metal layer thickness at all locations. Acontinuous operation is especially desirable in order to render possiblea minimal setting period.

In consequence, the basic object of the present invention is to avoidthe disadvantages of prior art and, more especially, to provide anapparatus for electrolytically treating a board-like item to be treated,by means of which apparatus the boards or foils can be continuouslyelectrolytically treated, and the apparatus can also be adapted tochanging formats/widths of the boards or foils to be treated at any timerapidly in an automatic manner or possibly in a manual manner, in orderto achieve an even distribution of the metal layer thickness on theboard or foil surfaces. During the continuous operation, the edge regionof the boards or foils, where prescribed layer thickness tolerancescannot be maintained, should be as small as possible. In addition,however, an extra requirement is to use as high an electrolytic currentas possible in order to permit the boards or foils to be electrolysedwith a short treatment time, so that a high throughput of the items tobe treated per unit of time becomes possible with the arrangement. Evenwith a large electrolytic current, the thickness of the metal layer onthe item to be treated is always to be kept as uniform as possible asfar as the edge region.

The apparatus according to the invention substantially constitutes acontinuous system, through which the item to be treated is guidable in aplane of conveyance in a substantially horizontal direction ofconveyance. Counter-electrodes are disposed in this continuous systemand are situated substantially parallel to one another opposite theplane of conveyance, and screens for shielding from high current densityfields in the edge region of the item to be treated are disposed betweenthe plane of conveyance and the counter-electrodes. The screens are eachin the form of at least two flat portions, which are disposedsubstantially parallel to each other, one portion of the screens beingdisposed so as to lie opposite the plane of conveyance, and the otherportion being disposed so as to lie opposite the counter-electrodes. Thescreens are mounted so as to be displaceable in one direction, whichextends substantially parallel to the plane of conveyance andsubstantially perpendicularly relative to the direction of conveyance.

Moreover, the continuous system has the additional features required forelectrolytic treatment, for example containers for accommodating thetreatment fluid, possibly nozzles, pumps and pipes for transferringfluid to the individual units, drive units for conveying the item to betreated through the system and guide means for support in the system, aswell as current sources, contacting means for the electrodes and theitem to be treated and current supply lines.

The method according to the invention is used for electrically shieldingedge regions of a board-shaped substrates item to be treated during theelectrolytic treatment thereof in the continuous system.

The apparatus and the method are suitable for the electrolytic treatmentof, more especially, printed circuit boards and conductor foils. Theitem to be treated can either be electrolytically metallised or beetched. In one case, the item to be treated is connected as the cathode,and the counter-electrodes are connected as the anodes, while the itemto be treated and the counter-electrodes in the other case are connectedwith transposed polarity.

The portion of the screens situated directly opposite the plane ofconveyance is disposed so as to be set back from the other portion ofthe screens by a spacing b towards the edge of the item to be treated.Such an arrangement renders possible a very effective shielding of theedge regions of the item to be treated from a concentration of the fieldlines. Without such an offset arrangement of the screen portions itwould not be possible to achieve an optimum even distribution of themetal layer thicknesses.

In order to adapt the screen arrangement particularly well to changingformats of the actual boards or foils to be treated, the first andsecond flat portions of the screens can be mounted so as to bedisplaceable independently of one another. The particular positions ofthe screen portions are determined by positions of the screen portionswhich are to be predetermined relative to the location of the anodes andof the traversing item to be treated, and said positions are set for thepassage of such formats.

In particular, the above-mentioned spacing b can be adjustable bysuitable structural elements. The spacings between the first and secondflat portions and the counter-electrode and the plane of conveyance arealso preferably adjustable independently of one another.

A controlled motor drive may be provided, for example, for the automaticsetting of the spacing b and the spacings between the flat portions andthe counter-electrode and the plane of conveyance, the screens beingdisplaceable by said drive.

In a preferred embodiment, the flat portions have edge regions, whichcan coincide with the item to be treated and the counter-electrodes,these edge regions having openings and/or recesses at the edges.

In order to permit relatively narrow boards to be effectively treatedalso, the flat portions should be made sufficiently wide, in thetransverse direction, relative to the direction of conveyance, that evena narrow item to be treated can be reliably screened in the edge region.

The counter-electrodes, which are preferably disposed on both sides ofthe plane of conveyance, are generally divided into a plurality ofpartial segments when viewed with respect to the direction of conveyancein the case of relatively long continuous systems, so as to ensure aseparate disconnection of the electrodes. This is necessary in order toavoid current flowing over counter-electrodes, since no item to betreated is situated opposite said counter-electrodes. If it were notpossible to disconnect the flow of current at these partial segments,the current density in the edge regions would be increased at the frontedges of the boards and foils, so that a metal layer with a thicknessoutside the required tolerance would be deposited there. When thecounter-electrodes are divided into segments in such manner, the screensaccording to the invention preferably have a length, which correspondsto the length of the partial segments of the counter-electrodes, whenviewed with respect to the direction of conveyance.

The invention is described more fully hereinafter with reference toFIGS. 1 to 3. In the drawing:

FIG. 1a is a cross-sectional view through a double screen and a layerthickness pattern obtained thereby;

FIG. 1b is a plan view of a double screen;

FIG. 2 is a plan view of edge regions of various flat portions ofscreens;

FIG. 3a is a schematic cross-sectional view through the electroplatingarrangement without a shielding screen according to prior art and alayer thickness pattern obtained thereby;

FIGS. 3b and 3 c are schematic cross-sectional views through screenarrangements according to prior art and layer thickness patternsobtained thereby.

FIG. 3a illustrates the conventionally obtained layer thickness patternon a board-like item to be treated in an electroplating arrangementwithout shielding screens according to prior art. FIGS. 3b and 3 cillustrate corresponding patterns when known screen arrangements areused with only one screen portion.

FIG. 3a illustrates the layer thickness d of a non-screened edge, forexample of a printed circuit board, after electroplating. A relativelayer thickness of d=1 is achieved in the central region of the blank,with a layer thickness of 0.03 mm, for example, in practical usage. Atthe same time, this layer thickness constitutes the desired layerthickness. In the cross-sectional view, the position of the item to betreated 1, for example a printed circuit board, relative to the anode 2is illustrated transversely relative to the direction of conveyance. Theelectrical contacting of the printed circuit board, which is notillustrated, is situated in the region 3 outside the Figure. Means forconveying and guiding the printed circuit boards in the continuouselectroplating system are also not illustrated in the Figure.

Because the layer thickness patterns on the underside of the printedcircuit board behave in a mirror-image manner relative to the patternson the upper side, only one anode is illustrated on one side. Theanode/cathode spacing, i.e. the distance between the anode surface andthe surface of the item to be treated, amounts, in practical usage, tobetween about 60 mm and about 120 mm in continuous electroplatingsystems. The layer thickness pattern 4, schematically sketched here,only reaches the relative desired layer thickness d=1 with a relativespacing a=1 from the lateral edge 5 of the printed circuit board. Theelectrical field lines are concentrated at the lateral edge of theprinted circuit board not only due to the field line density extendingfrom the anode surface protruding beyond the printed circuit boards, butalso due to the local current density being set in a region on theprinted circuit board. In consequence, an undesirable “layer thicknessdepression” 7 is formed there with a relative thickness of less thand=1. In the present case, the greatest layer thickness is formeddirectly at the lateral edge of the printed circuit board. In order toavoid so-called burning phenomena (pulverulent metal layer, which isgenerally formed when the cathodic current density is too high), thispeak current density must lie below a limit current density, where suchburning phenomena are produced. In consequence, a correspondingly lowermean current density must be set for the electroplating operation.

An electroplating arrangement is schematically illustrated in FIG. 3band has a flat screen 8, which is displaceably disposed in the vicinityof the anode 2. By using this screen, however, no changes occur in thebasic pattern of the layer thickness. In this case also, a “layerthickness depression” is produced, even if such is smaller than when thescreen is not used. However, the usable region of the printed circuitboard blank becomes greater as a result of this depression beingsmaller.

In the case of the arrangement of FIG. 3c, the usable region is aboutthe same size as in the case of the arrangement of FIG. 3b. Here, thescreen 8 is disposed close to the surface of the item to be treated 1.As the spacing from the printed circuit board becomes less, the screenedge 9 acts increasingly on the printed circuit board so as toconcentrate the field lines. In consequence, a layer thickness hump 10occurs at this location. This hump continues as far as the lateral edge5 of the printed circuit board. Even with this spacing between thescreen and the printed circuit board, the usable region of the printedcircuit board is not greater than in the case of the arrangement of FIG.3b.

Contrary thereto, the upper portion of FIG. 1a is a cross-sectional viewthrough the double screen 11 according to the invention. Said screencomprises a first flat portion 12, which is situated directly oppositethe counter-electrode (close to the anode), and a second flat portion13, which is situated directly opposite the item to be treated (close tothe cathode). The two flat portions are interconnected by means ofstructural elements 14, which are preferably formed from plasticsmaterial. These structural elements permit the positions of the firstand second screen portions 12 and 13 to be changed relative to eachother and in respect of their location relative to the anode andcathode. This means that the spacing 15 between the two flat portions aswell as the location of the front edges 16 and 17 of the two screenportions are adjustable. In consequence, the projecting length 18(spacing b) of the screen portion 12 beyond the portion 13 can be set.The double screen 11 is mounted in bearings 19 so as to be displaceabletransversely relative to the direction of conveyance.

A different structural solution can also be selected, of course, forsetting the screen portions relative to each other.

When the screen parameters are correctly set, the double screen 11compensates virtually fully for the “layer thickness depression” 7 andthe layer thickness hump 10 (see lower part of the illustration in FIG.1a). Adjustable screen parameters are the spacing 15, the projectinglength 18 (spacing b) and the position of the double screen in the spacebetween the anode and the item to be treated.

Since the spacing between the screen portion 12, close to the anode, andthe anode and the spacing between the screen portion 13, close to thecathode, and the item to be treated are advantageously no longer changedafter the assembly of the continuous system, the adjustment of thespacing 15 and of the projecting length 18 (spacing b) must generallyonly be effected once manually during the operation. A controllabledrive, which is not illustrated, causes the movement 20 of the doublescreen 11 and/or 12 transversely relative to the direction ofconveyance.

When only printed circuit board blanks with a constant width are beingprocessed, the screens only need to be set once by hand in order toachieve a specific covering 21 in the edge region of the printed circuitboards.

If the widths of the printed circuit boards are constantly changing,however, the printed circuit board covering is preferably set with amotor drive having known controlling and driving means.

The length of the screens transversely relative to the direction ofconveyance must be at least of such a magnitude that, even with thenarrowest printed circuit board blanks, an adequate screening of theedges is rendered possible.

Basically, it is also possible not to interconnect the screens 12 and 13securedly, but to displace them individually and singly transverselyrelative to the direction of conveyance. This is particularlyadvantageous when large current density differences from one item to betreated to the other are produced in an electroplating system. Thescreen 13, which is close to the cathode, can then be set in optimummanner to avoid edge burning phenomena.

Furthermore, FIG. 1b is a plan view of double screens 11 viewed towardsthe anodes 1. Bearings 19 for supporting the screens are disposedbetween the double screens and also carry the anodes. The anodes shouldbe segmented and electrically insulated from one another so that saidanodes can be disconnected individually when the item to be treatedenters the continuous system in the direction of conveyance and when theitem to be treated emerges from the system This function is described inthe publication DE-P 39 39 681 A1. Reference is made to thispublication. The required width 22 of the double screen advantageouslydepends on the width of an anode segment. The direction of conveyance ofthe item to be treated is indicated by the arrow 23.

FIG. 2 illustrates examples for specific embodiments of the first andsecond flat portions 12 and 13. The edge effect is somewhat reduced byperforating the front regions of the screen portions. The provision of-recesses at the front edges 16, 17 also leads to the even distributionof the metal layer. Such a perforation and/or the provision of recessesare/is highly suitable for fine adjustment.

All of the measures according to the invention together cause the usableregion of a printed circuit board blank to extend to at least 12 mm atthe printed circuit board edge, in practical usage, even in fineconductor technology. An additional fine adjustment can be achieved bydisposing a third screen portion between the two flat screens 12 and 13.The provision of this additional screen portion is especiallyadvantageous and incurs minimal expenditure when printed circuit boardblanks with only one uniform width are being processed in the continuoussystem. A motor drive for adapting the position of the screen portionsto changing formats is eliminated in this case.

All of the disclosed features as well as combinations of the disclosedfeatures are the subject-matter of this invention, provided that suchare not expressly indicated as being known.

List of Reference Numerals:

1. substrate to be treated (for example printed circuit board)

2. counter-electrode (for example anode)

3. contacting region

4. layer thickness pattern

5. printed circuit board edge

6. projecting counter-electrode region

7. “layer thickness depression”

8. single screen according to prior art

9. front edge of the single screen 8

10. layer thickness hump

11. double screen

12. screen portion situated directly opposite the counter-electrode(close to anode)

13. screen portion situated directly opposite the item to be treated(close to cathode)

14. structural elements for adjusting the screen portions 12 and 13

15. spacing between the screen portions 12 and 13

16. front edge of the screen portion 12

17. front edge of the screen portion 13

18. projecting length of the screen 12, situated opposite thecounter-electrode, beyond the screen 13 situated opposite the substrateto be treated (spacing b)

19. bearing for supporting the double screen

20. screen movement

21. printed circuit board covering

22. screen width

23. direction of conveyance

What is claimed is:
 1. Apparatus for electrolytically treating aboard-shaped substrate item to be treated in a continuous system,through which the item to be treated is guidable in a plane ofconveyance in a substantially horizontal direction of conveyance, theapparatus a. having counter-electrodes, which are situated substantiallyparallel to one another opposite the plane of conveyance, and b. havingscreens for shielding from high current density fields in the edgeregion of the substrate to be treated, said screens being disposedbetween the plane of conveyance and the counter-electrodes, c. thescreens each being in the form of at least two first and second flatportions, which are disposed substantially parallel to each other, saidsecond portion of the screens being disposed so as to lie opposite theplane of conveyance, and the said first portion being disposed so as tolie opposite the counter-electrodes, and d. the screens being mounted soas to be displaceable in a direction, which extends substantiallyparallel to the plane of conveyance and substantially perpendicularlyrelative to the direction of conveyance.
 2. Apparatus according to claim1, characterised in that the second portion of the screens situateddirectly opposite the plane of conveyance is disposed so as to be setback from the first portion by a spacing b towards the edge of the itemto be treated.
 3. Apparatus according to one of the preceding claims,characterised in that the said first and second flat portions aremounted so as to be displaceable independently of each other. 4.Apparatus according to one of claims 1-2, characterised in that thespacing b is adjustable by suitable structural elements.
 5. Apparatusaccording to one of claims 1-2, characterised in that the spacingsbetween the first and second flat portions and the counter-electrode andthe plane of conveyance are adjustable independently of one another. 6.Apparatus according to one of claims 1-2, characterised in that acontrollable motor drive is provided, whereby the screens aredisplaceable.
 7. Apparatus according to one of claims 1-2, characterisedin that the first and second flat portions have edge regions, which cancoincide with the item to be treated and the counter-electrodes, theseedge regions having openings and/or recesses at the edges.
 8. Apparatusaccording to one of claims 1-2, characterised in that the first andsecond flat portions are sufficiently wide, in the transverse direction,relative to the direction of conveyance, permitting a narrow substrateto be treated to be reliably screened in the edge region.
 9. Apparatusaccording to one of claims 1-2, characterised in that thecounter-electrodes, when viewed with respect to the direction ofconveyance, are divided into a plurality of partial segments, and thescreens have a length, which corresponds to the length of the partialsegments of the counter-electrodes, when viewed with respect to thedirection of conveyance.
 10. Method of electrically shielding edgeregions of a board-shaped substrate to be treated during theelectrolytic treatment thereof in a continuous system, through which thesubstrate to be treated is guided in a plane of conveyance in asubstantially horizontal direction of conveyance, a. the screens forshielding from high current density fields in the edge region of thesubstrate to be treated being disposed between the plane of conveyanceand counter-electrodes, which are situated substantially parallel to oneanother opposite the plane of conveyance, b. the screens also each beingin the form of at least two first and second flat portions, which aredisposed substantially parallel to each other, second portion of thescreens being disposed so as to lie opposite the plane of conveyance,and the first portion being disposed so as to lie opposite thecounter-electrodes, and c. the screens being mounted so as to bedisplaceable in a direction, which extends substantially parallel to theplane of conveyance and perpendicularly relative to the direction ofconveyance.